Seeing as "24" is edited and finished on an Avid there's a good chance you can do the split screens the same way... I would even say "exactly" the same way!
They use PIP (picture in picture) to create all the split screen effects.
As far as other programs, Magic bullet is nice for the preset color effects. I use fluidfilm that comes with Xpress pro to create 24p from 30i. Photoshop and After GenArts for plug-ins. Others may have their personal choices as well.
I have another problem, maybe you can help me with?
What is the best way to drag a frame out of a movie (25p in Avid) and edit it in Photoshop and then drag it back into Avid?
I did it but the size of the frame changes dramatically... I wonder what I am doing wrong?